1. Field of the Present Invention
The present invention relates generally to apparatus for superimposing a signal on another signal, and more particularly, to an apparatus used in video equipments to overlay external information on a video signal.
2. Description of the Related Art
Apparatus for interposing information such as titles on a picture have been used in built-in-camera VTR (Video Tape Recorder) and the like. FIG. 1 is a block diagram showing a typical example of a conventional apparatus of this kind. Referring to FIG. 1, the conventional apparatus includes an A/D (Analog/Digital) converter 12 which from a terminal 10 receives a luminance signal (hereinafter referred to as Y signal) such as a video signal produced by a video camera or reproduced by a VTR to digitize the same based on a certain signal level and converts the same into digital serial data; a shift register 16 connected to the A/D converter 12 and a terminal 14 for converting the serial data and parallel data, for example, of 8 bits into each other in synchronization with a clock signal supplied from the terminal 14; a timing generator 22 connected to the terminals 14, 18 and 20 for generating an address signal showing a position on an image of a portion being processed of the video signal, and in accordance with the processing mode, generating a signal indicating operation timing for elements included in the superimposing apparatus in response to a horizontal synchronizing signal H.sub.D and a vertical synchronizing signal V.sub.D of the video signal inputted from the terminals 18 and 20; a memorv 24 connected to the shift register 16 and the timing generator 22 for storing the parallel data converted by the shift register 16 in the address selected by the address signal from the timing generator 22 and for outputting the stored contents of the address selected by the address signal from the timing generator 22 to the shift register 16; a color synthesis circuit 26 connected to the shift register 16 for outputting a component signal for different colors of RGB (Red-Green-Blue) for displaying the title in multi-color in response to a signal which is read out from the memory 24 and converted into the serial data by the shift register 16, in accordance with the colors specified by a controller (not shown) or the like; and a display circuit 32 for mixing each of the component signals of RGB and a video signal inputted from a terminal 28 to be superimposed and outputting the mixed signals to an output terminal 30.
Each of the signals produced by the timing generator 22 includes a writing control signal WE for controlling a timing of writing the data to the memory 24 and a reading control signal OE for controlling a timing of reading the data from the memory 24.
The display circuit 32 includes a matrix circuit for synthesizing, for example, a Y signal and color difference signals of the title from the component signal; a circuit for producing a luminance blanking signal and a chroma blanking signal showing a timing of switching between the Y and the color signals of the title and those of the picture to be superimposed respectively; and a switching circuit responsive to the blanking signal for switching between the signal of the title and the signal of the picture at a high speed.
Referring to FIG. 1, an operation of a conventional superimposing apparatus will be described. The apparatus has two operation modes. The first is a title-image storing mode for reading a title image and storing the same into the memory 24. The second mode is a superimposing mode for overlaying the title stored in the memory 24 on a video signal inputted from the terminal 28. The modes are switched to each other by an operator by means of an operation panel (not shown).
In the title-image storing mode, a Y signal of a video signal showing a title taken by a camera is inputted from the terminal 10. Inputting the title is carried out by, for example, hand-writing the title on a white paper and converting the title into video signals by a video-camera or the like. The A/D converter 12 compares a level of the Y signal with a given level of a reference signal to convert the Y signal into the digital data by outputting a value of "0" when the level of the Y signal is higher than that of the reference signal and otherwise outputting a value of "1".
The digital data is inputted to the shift register 16. The shift register 16 shifts and stores 8 bits of the serial data at a time and outputs the same as the parallel data of 8 bits to the memory 24. H.sub.D and V.sub.D, inputted to the timing generator 22, synchronize with the Y signal inputted from the terminal 10. The synchronization is carried out by extracting only a sync signal from the video signal outputted from a video camera taking a title and amplifying the same.
The timing generator 22 outputs a signal showing a vertical and horizontal positions of the Y signal being processed on the picture frame in response to the clock, H.sub.D and V.sub.D. The signal indicates an address in which the data is stored in the memory 24.
When storing all the data of the title image inputted from the terminal 10 in the memory 24 is finished, a hand-written title image is stored in the memory 24 as the image represented by two different values each representing a background portion and a title portion, respectively.
Now, the apparatus operates as follows when the title image stored in the memory 24 is superimposed on a video signal inputted from the terminal 28.
A video signal taken by, for example, a video camera, or reproduced by a VTR is inputted from the terminal 28.
The horizontal synchronizing signal H.sub.D and the vertical synchronizing signal V.sub.D of the video signal inputted from the terminal 28 are inputted to the terminals 18 and 20 respectively. The timing generator 22 outputs an address signal representing a position of a pixel on the picture frame and a OE signal indicating the timing of the reading from the memory 24 in response to the clock, H.sub.D and V.sub.D. The contents of the address specified by the address signal are outputted to the shift register 16.
The shift register 16 converts the parallel data of 8 bits read out from the memory 24 into the 8-bit serial data by serially shifting and outputting the same.
The color synthesis circuit 26 outputs a component signal of RGB for displaying a title in a color specified by the operator in response to the serial data from the shift register 16.
The display circuit 32 mixes the above-described component signals and the video signal from the terminal 28 and outputs the mixed signals to the output terminal 30. On this occasion, the display circuit 32 switches the blanking signals at a high speed in response to those signals such that an image of the title is outputted for the part where the title characters exist and video signals are outputted for the rest of the part.
A signal outputted from the output terminal 30 is recorded on, for example, a video tape. The recorded signal is reproduced by a home television or the like.
For displaying the title stored in the memory 24, various functions are provided such as improving fineness of the display, reversing the title character and the background, displaying the title in a desired color, so-called "wipe" for displaying and/or erasing the title from a predetermined position or in a plurality of directions, "scrolling" for parallel-transferring the entire title picture on the frame and the like. In some cases, memories capable of storing the data of a plurality of frames for the title characters are provided. In some cases, the titles can be changed from one to another.
As described above, a conventional superimposing apparatus is very convenient which provides an easy and real video shooting and/or a video editing. However, there still exist such problems to be solved as the following.
Conventionally, a title picture is formed by hand-writing characters on paper, a white board or the like and converting the same into video signals. The title is represented by two different values with reference to luminance levels of character or pattern portions and a background. If contrast of an object is not sufficient or the background portion is spotted, the title picture may not be reproduced satisfactorily. In many cases it results in reduced sharpness of an image of a character profile. In accordance with a conventional method, hand-written characters can be inputted with relative ease. However, printing characters cannot be easily made a title. Even though some titles are used repeatedly, it is necessary to make a new one each time. Namely, the conventional apparatus has a problem in its operability and quality of a superimposed picture.